Orbital tensioner assembly

ABSTRACT

In an aspect, a tensioner is provided for tensioning an endless drive member that is engaged with a rotary drive member on a shaft of a motive device. The tensioner includes a base that is mountable to the motive device, a ring that is rotatably supported by the base in surrounding relationship with the shaft of the motive device and which is rotatable about a ring axis, a tensioner arm pivotally mounted to the ring for pivotal movement about an arm pivot axis, and first and second tensioner pulleys. The first tensioner pulley is rotatably mounted to the tensioner arm. The tensioner arm is biased towards a first span of the endless drive member on one side of the rotary drive member. The second tensioner pulley is rotatably mounted at least indirectly to the ring and is biased towards a second span of the endless drive member on another side of the rotary drive member. The ring is rotatable in response to hub loads in the first and second tensioner pulleys that result from engagement with the first and second spans of the endless drive member.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of PCT applicationPCT/CA2013/001085 and PCT/CA2013/001040, the disclosures of both ofwhich are incorporated herein by reference in their entirety.

FIELD

This disclosure relates to tensioners for endless drive members and inparticular to a tensioner that operates to tension an endless drivemember engaged by two separate motive devices such as an engine and amotor/generator unit.

BACKGROUND

It is common for vehicle engines to drive a plurality of accessoriesusing an accessory drive system that includes a belt. In some vehicles,a motive device is provided such as a motor/generator unit (MGU) thatcan be used for a number of purposes, such as, for example, driving oneor more accessories via the belt when the engine is temporarily offwhile the vehicle is stopped for a short period of time (e.g. at astoplight). Another purpose is for use as part of a belt alternatorstart (BAS) drive system wherein the MGU is used to start the enginethrough the belt. Another purpose is to supply additional power to theengine when needed (e.g. when the vehicle is under hard acceleration).In such situations special tensioning devices are typically needed toensure that the belt is under the appropriate amount of tensionregardless of whether it is being driven by the MGU or by the engine. Inmany instances however such tensioning devices are not optimal andresult in relatively high belt tensions and hub loads on the variouspulleys in the system, thereby negatively impacting fuel economy andcomponent life.

It would be desirable to provide a tensioning system that at leastpartially addresses one or more of the problems described above andother problems.

SUMMARY

In an aspect, a tensioner is provided for tensioning an endless drivemember that is engaged with a rotary drive member on a shaft of a motivedevice. The tensioner includes a base that is mountable to the motivedevice, a ring that is rotatably supported by the base in surroundingrelationship with the shaft of the motive device and which is rotatableabout a ring axis, a tensioner arm pivotally mounted to the ring forpivotal movement about an arm pivot axis, and first and second tensionerpulleys. The first tensioner pulley is rotatably mounted to thetensioner arm. The tensioner arm is biased towards a first span of theendless drive member on one side of the rotary drive member. The secondtensioner pulley is rotatably mounted at least indirectly to the ringand is biased towards a second span of the endless drive member onanother side of the rotary drive member. The ring is rotatable inresponse to hub loads in the first and second tensioner pulleys thatresult from engagement with the first and second spans of the endlessdrive member.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other aspects of the disclosure will be more readilyappreciated by reference to the accompanying drawings, wherein:

FIG. 1 is a side view of an engine containing a tensioner in accordancewith an embodiment of the present disclosure;

FIGS. 2 and 3 are perspective views of the tensioner shown in FIG. 1;

FIGS. 4 and 5 are perspective exploded views of a variant of thetensioner shown in FIG. 2;

FIG. 6 is a sectional perspective view of a variant of the tensionershown in FIG. 2;

FIG. 7 is a perspective exploded view of the tensioner shown in FIG. 6;

FIG. 8 is a sectional perspective view of a portion of the tensionershown in FIG. 2;

FIG. 9 is a perspective exploded view of a variant of the tensionershown in FIG. 2, incorporating different damping structures;

FIG. 10 is a sectional side view of the tensioner shown in FIG. 9;

FIG. 11 is another sectional side view of the tensioner shown in FIG. 9;

FIG. 12 is a perspective view of another variant to the tensioner shownin FIG. 2

FIGS. 13-17 are perspective views of other variants of the tensionershown in FIG. 2;

FIGS. 18-20 are schematic views of the tensioner shown in FIG. 2 underdifferent operating conditions;

FIGS. 21 and 22 are schematic views of different engine and accessorylayouts connected by an endless drive member and incorporating thetensioner shown in FIG. 2; and

FIGS. 23 and 24 are schematic views of the tensioner shown in FIG. 2 indifferent positions relative to a motive device to which it is mounted;

FIG. 25 is an exploded view of a tensioner in accordance with anotherembodiment of the disclosure;

FIG. 26 is a sectional elevation view of the tensioner shown in FIG. 25;

FIG. 27 is an exploded sectional elevation view of a portion of thetensioner shown in FIG. 25;

FIG. 28 is a non-exploded sectional elevation view of the portion of thetensioner shown in FIG. 27;

FIG. 29 is an engine layout illustrating the forces on the tensionershown in FIG. 25;

FIG. 30 is a sectional elevation view similar to FIG. 26, but showingforces acting on bushings supporting a ring of the tensioner;

FIG. 31 is another sectional elevation view that shows forces acting onbushings supporting a tensioner arm from the tensioner shown in FIG. 25;and

FIG. 32 is another sectional elevation view that shows forces acting onanother portion of the tensioner shown in FIG. 25.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

Reference is made to FIG. 1, which shows a crankshaft 910 from an engine913 from a vehicle (not shown). It will be noted that the engine 913 isshown as a simple rectangle for illustrative purposes. It will beunderstood that the engine 913 may have any suitable shape and may beany suitable type of engine such as a spark-ignition engine or a dieselengine. The vehicle may be any suitable vehicle, such as an automobile,a truck, a van, a minivan, a bus, an SUV, a military vehicle, a boat orany other suitable vehicle.

The crankshaft 910 has a crankshaft pulley 912 thereon. The crankshaftpulley 912 drives one or more vehicle accessories via a belt 914. Theterm ‘belt’ is used herein for convenience, however for the purpose ofthe claims and for the scope of this disclosure it will be understoodthat the belt 914 may alternatively be any other type of suitableendless drive member. It will further be noted that, in cases where theendless drive member is a belt, it may be any suitable type of belt,such as a flat belt, a V belt, a poly-V belt, a timing belt, or anyother suitable type of belt. The term ‘pulley’ is similarly used forconvenience and any other suitable rotary drive member may be usedinstead, such as a sprocket.

The accessories may include, for example, the MGU 916, an airconditioning compressor 918, a water pump 920, a power steering pump 921and/or any other suitable accessories. The system further includes aplurality of idlers 925 that are positioned to provide a selected amountof belt wrap about the pulleys of some of the accessories.

Each of the driven accessories has a shaft, and a pulley that may beconnectable and disconnectable from the shaft via a clutch. For example,the MGU shaft, clutch and pulley are shown at 950, 952 and 954respectively. In another example, the air conditioning compressor shaft,clutch and pulley are shown at 956, 958 and 960 respectively. Clutchingeach of the accessories permits each to be disconnected when not neededwhile the belt 914 itself is still being driven by the crankshaft 910.

In some vehicles, such as some hybrid vehicles, the engine 913 may bestopped temporarily in some situations (such as when the vehicle isstopped at a stoplight) and may then be started again when it is timefor the vehicle to move. In such situations, the MGU 916 can be operatedas a generator when the engine 913 is running so as to generateelectricity for storage in a vehicle battery (not shown). In someembodiments, the MGU 916 operated as an electric motor to drive thecrankshaft 912 via the belt 914, enabling the engine 913 to be startedvia the belt 914 (i.e. a belt-alternator start (BAS) drive system).

The MGU 916 may instead be some other type of motive device such as anelectric, hydraulic or pneumatic motor, which may be used to driveaccessories or to start the engine 913. The MGU or other motive device916 may be referred to generally as a supplemental motive device, as itis a supplemental means for driving the belt 914, whereas the engine 913is a primary motive device for driving the belt 914. Furthermore, insome embodiments, the engine 913 may instead be some other type ofmotive device, such as an electric motor. Instead of, or in addition to,being used to start the engine 913 and/or to drive accessories while theengine 913 is off, the supplemental motive device may be used to providea power boost to the engine 913 via the belt 914 (e.g. to provide aburst of acceleration for the vehicle).

Providing tension in the belt 914 is beneficial in that it reduces theamount of slip that can occur between the belt 914 and the drivenaccessory pulleys, between the belt 914 and the MGU 916, and between thebelt 914 and the crankshaft 910. In FIG. 1, the direction of rotation ofthe crankshaft 910 is shown at DIR1. When the engine 913 is driving thebelt 914 and the MGU 916 acts as a generator, it will be understood thata relatively higher tension will exist on the trailing belt span, shownat 914 a, and a relatively lower tension will exist on the leading beltspan, shown at 914 b, where the terms ‘trailing’ and ‘leading’ arerelative to the crankshaft pulley 912 in this context. In general, thebelt tension will decrease progressively through each belt span alongthe belt routing between the span 914 a and the span 914 b. By contrast,when the MGU 916 is driving the belt 914 and the engine 913 is off, thetrailing belt span, shown at 914 c (trailing relative to the MGU 916)has the highest belt tension, and the leading belt span 914 d (leadingrelative to the MGU 916) has the lowest belt tension. Thus it can beseen that the belt tension in the spans 914 c and 914 d can varysignificantly during operation of the vehicle in the two different modes(i.e. in a first mode where the engine 913 is the sole driver of thebelt 914 as compared to a second mode where the MGU 916 is the soledriver of the belt 914). Tensioners have been proposed for some vehiclesin which the tensioner has two arms that are fixedly connected to oneanother to form a V, wherein each arm has a pulley, and wherein the V ispivotally mounted to a base that is fixedly mounted to a region of theengine inside the contained area of the belt. The pulleys engage twodifferent spans of the belt, (e.g. a span on either side of an accessorysuch as an MGU). As a result of their configuration, such a tensioner iscapable of maintaining tension in both spans so that the belt tension iskept in the span needing it the most, regardless of whether the MGU isbeing driven as a generator or is being operated as a motor.

Such a tensioner, however, can be bulky and there is not alwayssufficient room in the aforementioned region to locate it.

In accordance with an embodiment of the present invention, an orbitaltensioner 10 is provided for tensioning the endless drive member 614,which is engaged with the rotary drive member 954 on the shaft 950 ofthe motive device 916. With reference to FIGS. 2 and 3, the tensioner 10includes a base 12, a ring 14, a tensioner arm 16, a first tensionerpulley 18 and a second tensioner pulley 20.

The base 12 may be made from aluminum or some other suitably strongmaterial and is fixedly mountable to the MGU 916. In the example shownin FIG. 2 the base 12 includes a plurality of fastener apertures 22,which receive fasteners 24 (FIG. 1) for mounting the base 12 to ahousing of the MGU 916.

The ring 14 may also be made from aluminum or another suitable materialand is rotatably supported by the base 12 in surrounding relationshipwith the shaft 950 of the motive device 916 and is rotatable about aring axis shown at A_(R) in FIGS. 1 and 2. As shown in FIG. 1, the ringaxis A_(R) may be coaxial with the axis of rotation of the MGU shaft950, which is shown at A.

FIGS. 4 and 5 show exploded views of the tensioner 10. It will be notedthat the base 12 shown in FIGS. 4 and 5 is a minor variant of the baseshown in FIGS. 2 and 3, with a principal difference being a differentdistribution of mounting apertures 22. Referring to FIGS. 4 and 5 afirst ring bushing 26 and a second ring bushing 28 are provided. Thering bushings 26 and 28 can be configured to apply any desired amount offriction to the ring 14 to provide any desired amount of damping to themovement of the ring 14 on the base 12. When used intentionally to applya selected amount of damping to the movement of the ring 14, the ringbushings 26 and 28 may be referred to as first and second ring dampingmembers 26 and 28. Suitable material of construction for the bushings 26and 28 may be, for example, polyamide 4.6 or 6.6 or some other suitablepolymeric material.

In the embodiment shown, a clamping member 30 is provided and isconnected to the ring 14 such that the clamping member 30 cooperateswith the ring 14 to clamp the base 12 and the first and second ringdamping members while still permitting sliding movement of the ring 14relative to the base 12. With this arrangement, the first ring bushing26 is positioned between the clamping member 30 and a first face 32(FIG. 5) of the base 12, and the second ring bushing 28 is positionedbetween the ring 14 and the a second face 34 (FIG. 4) of the base 12.During movement of the ring 14 when the tensioner 10 is in use, thesliding occurs by the clamping member 30 on the first bushing 26 and/orby the first bushing 26 on the base 12, and sliding also occurs by thering 14 on the second bushing 28 and/or by the second bushing 28 on thebase 12. As a result of the aforementioned sliding movement, the firstand second ring bushings 26 and 28 apply a frictional force (i.e. adamping force) to the ring 14.

In the embodiment shown, the first ring bushing 26 is a complete circle,covering the entire circumference of the ring 14 and base 12. However,the second ring bushing 28 covers less than the entire circumference ofthe ring 14 and base 12 (and in the embodiments shown, less than 180degrees of arc). The second ring bushing 28 is positioned in a firstregion of the tensioner 10 that is outside of a second region that liesunder the belt 914 (FIG. 1). In the first region there is less of aheight constraint on the tensioner components, whereas in the secondregion there can be significant height constraint. The part of thecircumference of the ring 14 and base 12 where the second ring bushing28 is not routed is in the second region of the tensioner 10, so as helpkeep the height of the tensioner 10 sufficiently low to avoidinterference with the belt 914.

Optionally, the clamping member 30 may be threadably connected to thering 14 (e.g. via engagement of threaded fasteners 36 with threadedapertures 38 in the ring 14) so as to permit adjustment of a gap betweenthe clamping member 30 and the ring 14, and therefore adjustment of theclamping force therebetween. This permits adjustment of a damping forceexerted on the ring 14 via the first and second ring damping members 26and 28.

It will be noted that the first and second ring bushings 26 and 28 haveradially extending portions, shown at 40 respectively, which are theportions of the bushings 26 and 28 that act on the first and secondfaces 32 and 34 of the base 12. Additionally however, the bushings 26and 28 further include axially extending portions 41 (FIG. 5) that actbetween the radially outer face 42 of the ring 14 and the radially innerring-receiving wall 44 of the base 22.

A sectional side view of the tensioner 10 is shown in FIG. 6, however,in this view it can be seen that the first and second ring bushings 26and 28 are replaced by a single bushing 46 that includes a first portion48 (see also FIG. 7) that is similar to the first bushing 26 (FIG. 4)and acts between the clamping member 30 and the base 12, and a secondportion 50 that is similar to the second bushing 28 and acts between thering 14 and the base 12.

Referring to FIG. 2, the tensioner arm 16 is made from aluminum oranother suitable material and is pivotally mounted to the ring 14 forpivotal movement about an arm pivot axis A_(A). The tensioner arm 16 hasthe first tensioner pulley 18 rotatably mounted thereon, for rotationabout a first pulley axis A_(P1), which is spaced from the arm pivotaxis A_(A). Referring to FIG. 1, the tensioner arm 16 is biased in afree arm direction towards a first span 914 d of the endless drivemember 914 on one side of the rotary drive member 954. The tensioner arm16 may be biased in the free arm direction by a tensioner arm biasingmember 52 (FIGS. 4, 5 and 8). For example, the tensioner arm biasingmember 52 may be any suitable kind of biasing member, such as forexample, a torsion spring having a first end 54 (FIG. 4) that engages afirst drive wall 56 (FIG. 5) on the arm 16, and a second end 58 (FIG. 4)that engages a second drive wall in a spring housing portion 62 on thering 14.

The tensioner arm 16 is part of a tensioner arm assembly that furtherincludes a shaft member 74 which mounts (e.g. via threaded engagement)to the ring 14, a pivot bushing 76 that pivotally supports the tensionerarm 16 on the shaft member 74, and an optional damping structure 76 thatincludes a polymeric (e.g. unfilled (non-reinforced) nylon) tensionerarm damping member 78 and a metallic (e.g. steel) sleeve 80 that holdsthe damping member 78 and protects the damping member 78 against damagefrom engagement with the torsion spring 52. The damping member 78provides damping for the movement of the tensioner arm 16. Thecomponents of the tensioner arm assembly may be similar to the analogouscomponents described in PCT publication no. WO2013/059929, the contentsof which are incorporated herein in their entirety. The tensioner armassembly may alternatively be as described in patent publicationsEP0450620B1, DE20319886U1, and DE04010928C2, the contents of all ofwhich are incorporated herein by reference in their entirety.

Referring to FIG. 2, the second tensioner pulley 20 is rotatably mountedat least indirectly to the ring 14 for rotation about a second pulleyaxis A_(P2). In the embodiment shown in FIG. 2, the pulley 20 is mounteddirectly to the ring 14, via a fixed projection 64 on the ring 14.

The second tensioner pulley 20 is biased towards a second span 914 c ofthe endless drive member 914 on another side of the rotary drive member954. This biasing occurs by virtue of the forces transferred to the ring14 by the tensioner arm biasing member 52. More specifically, duringoperation of the tensioner 10, when the first pulley 18 is engaged withthe belt span 914 d, the belt span 914 d applies a hub load to the firstpulley 18. This hub load acts on the arm 16 through the pulley 18. Theforce on the arm 16 is transferred through the biasing member 52, andinto the ring 14 itself, urging the ring 14 to pivot about axis A_(R) inthe opposite rotational direction to the direction of pivoting of thearm 16. This force transfer into of the ring 14 urges the secondtensioner pulley 20 in a second free arm direction, into the second beltspan 914 c. Thus the ring 14 is rotatable about the ring axis A_(R) inresponse to hub loads in the first and second tensioner pulleys 18 and20 that result from engagement with the first and second spans 914 d and914 c of the endless drive member 914.

Each of the pulleys 18 and 20 may have the same construction. Forexample, each pulley 18, 20 may include a pulley body 66, a bearing 68,and a pulley mounting fastener 72 used to mount (e.g. by threadedengagement) the pulley 18, 20 to the tensioner arm 16 or to theprojection 64. Optional first and second dust shields 70 are provided toprotect the bearing 68 from dust during operation of the tensioner 10.The dust shields 70 may be separate components that sandwich the bearing68 to inhibit the migration of dust and debris into the bearing 68. Ascan be seen one of the dust shields 70 for the pulley 18 is provided asan integral portion of the tensioner arm 16.

The bearing 68 may be a ball bearing, as shown, or it may be any othersuitable type of bearing. The bearing 68 could also be a bushing in someembodiments.

Reference is made to FIGS. 9, 10 and 11, which show the tensioner 10with some modified features. As can be seen in FIG. 9, the second ringbushing is shown at 128 and extends about the entire circumference ofthe ring 14 and the base 12. This provides improved stability of thering 14 in terms of resistance to yaw.

As can be seen in FIGS. 9 and 11, a different damping structure is usedto provide damping for the tensioner arm 16. The damping structure isshown at 82 and includes a damping member 84, a support member 86 and adamping member biasing member 88. The damping member 84 may be made fromany suitable material such as a suitable polymeric material, such aspolyamide 4.6 or polyamide 6.6. The damping member 84 slidingly engagesa damping surface 90 on the tensioner arm 16 (FIG. 11). A support member86 supports the damping member 84, and the biasing member 88 actsbetween a support surface 92 on the ring 14 and the support member 86.The biasing member 88 may be any suitable type of biasing member, suchas, for example, a steel Belleville spring washer. The support member 86may be made from a suitable material such as steel, so as to preventdamage (e.g. gouging) to the damping member 84 by the biasing member 88due to the relative softness of the damping member 84 as compared to thebiasing member 88.

The damping structure 82 may be similar to the damping structuredisclosed in US patent application publication US2008/0280713, thecontents of which are incorporated herein in their entirety. Providing adamping structure similar to the damping structure 82 is advantageous inembodiments where it is desirable to providing damping to the movementof the tensioner arm 16 that is independent of the hub load incurred bythe first pulley 18.

Another difference between the embodiments shown in FIGS. 2-8 and theembodiment shown in FIG. 9 is that the clamping member 30 in FIG. 9 isnot threaded, but instead includes clip portions that clip ontoreceiving members on the ring 14. In the embodiment shown in FIG. 9, theflange portion of the clamping member 30 (which is shown at 89) may berelatively thin in cross-section so as to render it resilient, and maybe shaped to apply a spring force on the damping member 26. Thisarrangement can be configured so that a consistent force is applied tothe damping member 26 by the clamping member 30 reducing the need forassembly worker expertise.

It will be further noted that the damping members 26 and 28 also providedamping that is substantially independent of the hub load incurred bythe pulleys 18 and 20. Additionally, it will be noted that the use oftwo damping members 26 and 28 both of which are at relatively largediameters (i.e. large moment arms) from the ring axis A_(R), reduces theaverage amount of force that each damping member 26 and 28 must apply toachieve a selected damping load.

The damping members 26 and 28 may have surface properties that providesymmetric damping in the sense that the damping force exerted by thedamping members 26 and 28 may the same irrespective of the direction ofmovement of the ring 14. Alternatively, however, the damping members 26and 28 may be provided with surface properties (e.g. a fish-scaleeffect) that provides lower damping in one direction and higher dampingin the opposite direction. Other means for achieving asymmetricaldamping are alternatively possible, such as the use of a ramp structurewhereby the ring 14 rides up the ramp structure urging it intoprogressively stronger engagement with a damping member (so as toincrease the damping force) during rotation in a first direction andwherein the ring 14 rides down the ramp structure urging it into weakerengagement with the damping member thereby reducing the damping forceduring movement in the second direction.

In other embodiments, the members 26 and 28 may be configured to provideas little damping as possible thereby increasing the responsiveness ofthe tensioner 10.

Reference is made to FIGS. 4, 5 and 12, which show an installation pin92 that facilitates installation of the belt 914 (FIG. 1) on the pulleys18, 20 and 954 when the tensioner 10 is already installed on the motivedevice 916. The installation pin 92 can be passed through an optionaltensioner arm pin aperture 94 into an optional ring pin aperture 96, tolock the tensioner arm 16 in a position that is away from a free armstop position and that is towards a load stop position. The free armstop position represents one end of a range of movement of the arm 16,and is the position that the tensioner arm 16 would end up in if therewere no belt present to resist the arm's movement. The load stopposition represents the other end of the range of movement of the arm 16and is the position the arm 16 would end up in if the belt tension weresufficiently high to completely overcome the biasing force of thebiasing member 52.

Once the belt 914 (FIG. 1) has been installed throughout the accessorydrive system on the engine 913, the installation pin 92 (FIG. 12) may beremoved from the apertures 94 and 96 permitting the biasing member 52 todrive the arm 16 and pulley 18 into the belt 914. The pin 92 is shown inthe installed position in FIG. 2. The installation pin 92 is a pin inthe examples shown herein, however it will be understood that theinstallation pin 92 may instead be any suitable type of arm lockingmember that locks the tensioner arm 16 in a selected angular position topermit the belt 914 to be installed on the pulleys 18, 20 and 954.

Reference is made to FIG. 13, which shows the tensioner 10 with anothervariation in features. In this embodiment, the tensioner 10 includes athird tensioner pulley shown at 21. The third pulley 21 permits thetensioner 10 have a selected amount of belt wrap about the MGU pulley954 (FIG. 1) while providing a selected orientation to the belt span 914c. As a result, the belt span 914 c can be routed to avoid interferencehazards near the engine 913.

Reference is made to FIG. 14, which shows the tensioner 10 with a secondtensioner arm assembly. In other words, the tensioner arm 16 is a firsttensioner arm, and the tensioner biasing member 52 is a first tensionerbiasing member, and the tensioner 10 includes a second tensioner arm 16′that may be similar but a mirror image of the first tensioner arm 16,and which is biased in a free arm direction into the belt span 914 c bya second tensioner biasing member 52′. The introduction of the secondbiasing member 52′ introduces a different set of forces into the ring 14during changes in belt tension and therefore changes in the hub loads onthe pulleys 18 and 20 than exists with the arrangement shown in FIGS. 2and 3.

Reference is made to FIG. 15, which shows the tensioner 10 that uses anarcuate, helical compression spring 152 instead of a torsion spring asthe tensioner biasing member. The compression spring 152 has a first end154 that engages a first drive surface 156 on the tensioner arm 16, anda second end 158 that engages a second drive surface 159 on the ring 14.

The tensioner 10 as shown in FIG. 16 includes two helical compressionsprings, shown respectively at 152 and 152′, which act on first andsecond tensioner arms 16 and 16′ respectively. In the embodiment shownin FIG. 16, each compression spring 152, 152′ acts between a respectivetensioner arm 16 and a drive surface on the ring 14. By contrast, anembodiment shown in FIG. 17 includes a single spring that acts betweenthe first and second tensioner arms 16 and 16′ and does not act directlyon the ring 14.

FIGS. 18-20 are schematic views of the tensioner 10 with a singletensioner arm 16, illustrating different situations. FIG. 18 illustratesa situation where the engine 913 (FIG. 1) is operating at asubstantially constant load, (e.g. at idle with no MGU load). The belttension in spans 914 c and 914 d may be substantially the same. FIG. 19illustrates a situation where the MGU pulley 954 is driven by the MGU 16(FIG. 1), either to operate accessories when the engine 913 is off, orto start the engine 913, or to provide a boost of power to a runningengine 913. As can be seen, the ring 14 has rotated clockwise as aresult of the increased tension in belt span 914 d and the reduced belttension in span 914 c. FIG. 20 illustrates a situation where the engine913 (FIG. 1) is under high load, thereby increasing the belt tension inspan 914 c and reducing the belt tension in span 914 d, while the MGU916 is either not operating or is operating as a generator. As can beseen the ring 14 has rotated counterclockwise as a result of the reducedtension in span 914 d and the increased tension in span 914 c.

FIG. 21 shows an alternative engine layout that includes an accessorypulley 970 (in this instance for a water pump) and two idlers 925 thatensure that there is a selected amount of belt wrap around thecrankshaft pulley 912 and around the accessory pulley 970 even when theMGU pulley 954 is being driven by the MGU 916 (FIG. 1). This reduces thelikelihood of slip at the crankshaft pulley 912 when the crankshaftpulley 912 represents a high load (e.g. during a BAS starting event).

FIG. 22 shows another alternative engine layout that includes only theMGU pulley 954, the crankshaft pulley 912, the tensioner 10 and an idler925.

FIGS. 23 and 24 illustrate an engine layout similar to that shown inFIGS. 18-20, but where the ring axis A_(R) is not co-axial with the axisA_(S) of the MGU shaft. FIG. 23 illustrates a situation where the ringaxis AR is ‘inboard’ of the shaft axis AS, while in FIG. 24 the ringaxis AR is outboard of the shaft axis AS. The terms ‘inboard’ and‘outboard’ are used here to indicate position relative to the region ofthe engine 913 that is contained within the belt 914 (shown at 98).While FIGS. 23 and 24 illustrate embodiments in which the ring axis ARis not coaxial with the shaft axis AS, it can be seen that the ring 14still surrounds the shaft 950.

It will be understood that, in the embodiments shown herein, thetensioner arm 16 pivots about an arm pivot axis A_(A) that is offsetfrom the shaft axis A_(S) of the MGU 916. This has several advantages.Firstly, under certain conditions, such as low frequency events such asa BAS starting event, the offset pivot axis of the tensioner arm 916 andthe use of a ring 14 that can have a relatively high inertia and thatmoves along a relatively large diameter path can control the movement ofthe tensioner 10 so as to reduce the likelihood of slip. Embodimentsthat incorporate two tensioner arms 16 and 16′ are advantageous in thatthey can more effectively filter out events (i.e. belt tensionfluctuations that are higher frequency), while providing an additionaldegree of freedom of movement which is the ring 14.

It has been found during testing of a tensioner in accordance with thepresent disclosure that the average belt tension and the average hubloads are lower than with some other types of tensioner. This results inmany advantages including: reduced fuel consumption during engineoperation, reduced belt wear (and therefore increased belt life), andreduced loads (and therefore reduced wear) on the pulleys and bearingsof the driven components such as for the air conditioning compressor,the water pump and the MGU itself. In particular, the reduced hub loadsapply to embodiments with a single tensioner arm 16, where the ring 14simply moves to a position to accommodate the belt tension acting on thefirst pulley 18 and the belt tension acting on the second pulley 20.

Another advantage of the embodiments described herein is that thetensioner 10 can be mounted to the MGU 916 to form a subassembly thatcan be installed in a vehicle relatively easily as compared to having anassembly line worker install the MGU, and then separately install atensioner system. This can reduce the overall cost to manufacture thevehicle by some amount.

A suitable sealing member may be provided between any suitable memberssuch as between the ring 14 and the base 12. The sealing member may befor example a skirt shield and/or one or more O-rings, a labyrinth sealor any other suitable type of seal to prevent the ingress of debris thatcould damage and/or jam the tensioner 10. Additionally, a suitablecoating can be provided on the rotatable ring to inhibit heat builduptherein from friction and/or to promote heat dissipation.

Reference is made to FIGS. 25 and 26, which show an embodiment of thetensioner 10 that is similar to that which is shown in FIGS. 9-11, butwith some additional modified features. The tensioner 10 in FIGS. 25-26is configured to generally resist tilting forces that would otherwisecause uneven wear and eventual wobbling and failure of the tensioner 10.As can be seen in FIG. 25, the second ring bushing 128 is provided,which extends about the entire circumference of the ring 14 and the base12 so as to provide improved stability of the ring 14 in terms ofresistance to yaw, as noted above in relation to FIGS. 9-11.

Additionally, as shown in FIG. 27, the flange 89 of the clamping member30 has a selected upward cant in a rest position and is resilient. Whenthe clamping member 30 is mounted to clamp the first and second ringbushings 26 and 128 (FIG. 28) the flange 89 is positioned to apply aselected force F4 on the bushings 26 and 128.

Referring to FIG. 29, an engine layout is shown that includes the MGUpulley 954, the crankshaft pulley 912, the tensioner 10 and an idler925. The forces acting in the plane shown in FIG. 29 include force F1that acts on the tensioner arm pulley 18, force F2 that acts on the ringpulley 20, and the reaction force F3 that provides a net zero resultantforce in the plane shown. The forces F1 and F2 are the result of thebelt 914 acting on the pulleys 18 and 20. The force F3 is, however, outof plane relative to the bushing 26, and as a result, the force F3generates a moment on the bushing 26, which is determined by the formulaM1=F3*L1, where M1 is the moment generated by the force F3, and L1 isthe distance between the vector axis of the force F3 and the verticalmidpoint between the bushings 26 and 128. The force F4 generated by theflange 89 of the clamping member 30 generates a moment that opposes themoment M1. In order to ensure that there is no tilting of the bushing 26in particular, the moment generated by the force F4 is to be greaterthan the moment M1. The moment generated by the force F4 is determinedby the formula M2=F4*r1, where M2 is the moment generated by the forceF4, and r1 is the radius at which the force F4 acts on the bushing 26.Thus by selecting suitable properties for the clamping member 30, thespring force F4 applied by the flange 89 may be selected to besufficient to ensure that no tilting occurs in the bushing 26, so as toimprove its operating life.

Instead of forming the flange 89 to apply a spring force on the bushing26, it is alternatively possible to provide a separate spring member,such as a wave ring (not shown) that can apply an axially directed forceon the bushing 26 to resist a tilting force.

Reference is made to FIG. 31. As can be seen in FIG. 31, the dampingmember biasing member 88 may be configured to apply a force F5 on thebushing 84, wherein the force F5 is selected to ensure that there is notilting force on the bushing 84 during operation of the tensioner 10.The force F2 acts on the bushing 84 to apply a moment determined by theformula M3=F2*L2, wherein the force moment M3 is the moment generated bythe force F2, and the distance L2 is the distance from the vector axisof the force F2 to the vertical midpoint between the two bushings 76 and84. To ensure that there is no net tilting force on the bushing 84, themoment generated by the force F5 is selected to be greater than themoment M3. The moment generated by the force F5 is determined by theformula M4=F5*r2, wherein the moment M4 is the moment generated by theforce F5 and the distance r2 is the radius at which the force F5 acts onthe bushing 84.

Reference is made to FIG. 32, which shows an alternative tensioner arm16 that can be used instead of the arm 16 shown in FIGS. 25-31. The arm16 shown in FIG. 32 is an arm-over-pulley configuration, in which thepulley 18 extends inwards towards the face of the alternator or MGU. Thearm 16 shown in FIGS. 25-31 is an arm-under-pulley configuration, inwhich the pulley 18 extends outwards from the arm 16 (i.e. away from thealternator or MGU). Aside from a difference in the particular distancesfor L2 and r2 in FIG. 32 relative to FIG. 31, the selecting of force F5to prevent a tilting force on the bushing 84 is substantially the same.

Based on the above description in relation to FIGS. 25-32, it can beseen that a force (i.e. force F4) generated by a spring member (e.g. theflange 89, or a wave ring) on a bushing (e.g. bushing 26) that supportsthe ring 14 on the base 12 can be selected to resist a tilting force(i.e. force F3) that results from the belt force on the pulleys 18 and20 during operation of the tensioner. Additionally, it can be seen thata force (i.e. force F5) generated by another spring member (e.g. aBelleville washer 88) on a bushing (e.g. bushing 84) that supports thetensioner arm 16 can be selected to resist a tilting force (i.e. forceF2) that results from the belt force on the tensioner arm pulley 18. Theforce F4 is selected depending at least in part on the force F3, thedistance L1 and the radius r1, and the force F5 is selected depending atleast in part on the force F2, the distance L2 and the radius r2.

Those skilled in the art will understand that a variety of othermodifications may be effected to the embodiments described hereinwithout departing from the scope of the appended claims.

The invention claimed is:
 1. A tensioner for tensioning an endless drivemember that is engaged with a rotary drive member on a shaft of a motivedevice, the tensioner comprising: a base that is mountable to the motivedevice; a ring that is rotatably supported by the base in surroundingrelationship with the shaft of the motive device and which is rotatableabout a ring axis; a tensioner arm pivotally mounted to the ring forpivotal movement about an arm pivot axis; a first tensioner pulleyrotatably mounted to the tensioner arm, wherein the tensioner arm isbiased towards a first span of the endless drive member on one side ofthe rotary drive member; and a second tensioner pulley that is rotatablymounted at least indirectly to the ring, wherein the second tensionerpulley is biased towards a second span of the endless drive member onanother side of the rotary drive member, wherein the ring is rotatablein response to hub loads in the first and second tensioner pulleys thatresult from engagement with the first and second spans of the endlessdrive member, wherein a force generated by a spring member on a bushingthat supports the ring on the base is selected to resist a tilting forcethat results from the belt force on the pulleys during operation of thetensioner.
 2. A tensioner as claimed in claim 1, wherein the secondtensioner pulley is an idler that is mounted to the ring for rotationabout a second pulley pivot axis that is fixed relative to the ring. 3.A tensioner as claimed in claim 1, wherein the tensioner arm is biasedin the free arm direction by a first biasing member which is a torsionspring.
 4. A tensioner as claimed in claim 1, wherein the tensioner armis biased towards the first span by a first biasing member which is afirst helical compression spring action between the tensioner arm andthe ring.
 5. A tensioner as claimed in claim 1, wherein the tensionerarm is a first tensioner arm and the second tensioner pulley is mountedon a second tensioner arm that is biased towards the second span of theendless drive member.
 6. A tensioner as claimed in claim 5, wherein thesecond tensioner arm is biased towards the second span by a secondbiasing member which is a second helical compression spring actionbetween the second tensioner arm and the ring.
 7. A tensioner as claimedin claim 1, further comprising a tensioner arm damping member positionedfor damping movement of the tensioner arm.
 8. A tensioner as claimed inclaim 1, wherein a ring damping member is engaged with the ring todamping movement of the ring.
 9. A tensioner as claimed in claim 8,wherein the ring damping member is configured to dampen movement of thering more strongly in a first rotational direction of the ring than in asecond rotational direction of the ring.
 10. A tensioner as claimed inclaim 8, wherein the ring damping member is a first ring damping memberthat is positioned between the ring and the base and engages a firstface of the base, and wherein a second ring damping member is connectedto the ring and engages a second face of the base.
 11. A tensioner asclaimed in claim 10, further comprising a clamping member that isconnected to the ring such that the clamping member cooperates with thering to clamp the base and the first and second ring damping memberswhile still permitting sliding movement of the ring relative to thebase.
 12. A tensioner as claimed in claim 11, wherein the clampingmember is threadably connected to the ring so as to permit adjustment ofa gap between a clamping member face and a ring race so as to permitadjustment of a damping force exerted on the ring via the first andsecond ring damping members.
 13. A tensioner as claimed in claim 1,further comprising an arm locking member configured to lock thetensioner arm in a selected angular position relative to the ring, suchthat the tensioner arm permits installation of the endless drive memberaround the first and second tensioner pulleys and the rotary drivemember.
 14. A tensioner as claimed in claim 1, wherein the ring axis iscoaxial with an axis of the shaft of the motive device.
 15. A tensioneras claimed in claim 1, wherein the ring axis is offset from an axis ofthe shaft of the motive device.
 16. A tensioner as claimed in claim 1,wherein a force generated by another spring member on a bushing thatsupports the tensioner arm is selected to resist a tilting force thatresults from the belt force on the tensioner arm pulley.